Modern gas turbines, especially aircraft engines, must satisfy the highest demands with respect to reliability, weight, power, economy, and operating service life. In the development of aircraft engines, the material selection, the search for new suitable materials, as well as the search for new production methods, among other things, play an important role in meeting standards and satisfying the demand.
The materials used for aircraft engines or other gas turbines include titanium alloys, nickel alloys (also called super alloys) and high strength steels. Titanium alloys are generally used for compressor parts, nickel alloys are suitable for the hot parts of the aircraft engine, and the high strength steels are used, for example, for compressor housings and turbine housings. The highly loaded or stressed gas turbine components, such as components for a compressor for example, are typically forged parts. Components for a turbine, on the other hand, are typically embodied as investment cast parts.
It is generally difficult to investment cast titanium and titanium alloys and similar reactive metals in conventional investment molds and achieve good results because of the metal's high affinity for elements such oxygen, nitrogen, and carbon. At elevated temperatures, titanium and its alloys can react with the mold facecoat. Any reaction between the molten alloy and the mold will result in a poor surface finish of the final casting which is caused by gas bubbles. In certain situations the gas bubbles effect the chemistry, microstructure, and properties of the final casting.
Once the final component is produced by casting, machining, or forging, further improvements in surface finish are typically necessary before it can be used in the final application. Asperities and pits on the surfaces of components can reduce aerodynamic performance in turbine blade applications, and increase wear/friction in rotating or reciprocating part applications.
In the case of titanium aluminide turbine blades, the cast airfoils may have regions in the dovetail, airfoil, or shroud that are cast/forged oversize. To machine these thin stock regions to the final dimensions, either mechanical machining (such as milling or grinding) or non-mechanical machining (such as electrochemical machining) are typically used. However, in either case, the costs of tooling and labor are high and result in manufacturing delays.
Moreover, the limited ductility and sensitivity to cracking of alloys, including titanium aluminide cast articles, may prevent the improvement of the surface finish of cast articles using conventional grinding and polishing techniques. Accordingly, there is a need for an intermetallic-based article for use in aerospace applications that has an improved surface finish and associated methods for manufacturing such an article.